This invention relates to the attachment of a methyl group on a carbon adjacent to an electron withdrawing group using dimethyl ether as the source of the methyl group. In another aspect it relates to the in situ conversion of the attached methyl group on a carbon adjacent to an electron withdrawing group to a double bond when oxygen or other oxidant is present with the dimethyl ether.
The attachment of a methyl group on a carbon adjacent to an electron withdrawing group or the formation of a carbon-carbon double bond is problematic at an industrial scale.
One approach is to form an enolate, that is, removing the proton on a carbon adjacent to an electron withdrawing group with a base to form a carbanion. When the electron withdrawing group contains a carbonyl group the combination of the carbanion and the carbonyl is an enolate. In organic chemistry alkylation of the enolate is done with a methyl halide such as methyl iodide.
Another approach reacts intermediates, such as propionic acid, propionic acid anhydride, or methyl propionate with formaldehyde or formaldehyde dimethylacetal or trioxane or paraformaldehyde to form methacrylic acid or methyl methacrylate. Although formaldehyde can be added in excess, the preferred modes of operation utilize the carboxylic acid intermediates in excess to avoid significant yield loss due to formaldehyde side reactions in the gas phase.
For example, in U.S. Pat. No. 4,736,062, Hagen et al. disclose a process of producing an alpha, beta-ethylenically unsaturated monocarboxylic acid compound which comprises the aldol-type condensation of a saturated aliphatic monocarboxylic acid and formaldehyde under vapor phase conditions in the presence of a hydrocarbon of 6 to 12 carbon atoms and a solid catalyst. This solid acid catalyst is described as comprising a cation of Group I or Group II metal and a silica support.
In U.S. Pat. No. 4,761,393, Baleiko et al. describe an in situ method for preparing an alkali metal ion-bearing particulate siliceous catalyst suitable for enhancing the vapor-phase condensation of a gaseous, saturated carboxylic acid with formaldehyde.
In U.S. Pat. No. 4,801,571 Montag et al. disclose a mixed oxide SiO.sub.2 --SnO.sub.2 --Cs ion catalyst and process for production of an alpha, beta-ethylenically unsaturated monocarboxylic acid by condensation of a saturated monocarboxylic acid with formaldehyde.
In U.S. Pat. No. 4,845,070, Montag describes a catalyst suitable for production of methacrylic acid by condensation of propionic acid with formaldehyde. The catalyst comprises a support which consists essentially of porous silica gel with cesium ions on the catalyst support surface, this support surface having a surface area of about 50 to about 150 M.sup.2 /g, a porosity of less than about 1 cm.sup.3 /gm, a pore size distribution such that less than about 10 percent of the pores present in the catalyst have a pore diameter greater than about 750 angstroms, and the cesium ions present in an amount of about 4 to about 10 percent by weight of the said catalyst.
In U.S. Pat. No. 4,942,258, Smith discloses a process for regeneration of a catalyst which comprises a support which consists essentially of porous silica with cesium ions on the catalyst support surface, said catalyst useful for production of methacrylic acid by condensation of propionic acid with formaldehyde.
In U.S. Pat. No. 5,710,328, Spivey et al. disclose a process for the preparation of alpha,beta-unsaturated carboxylic acids and the corresponding anhydrides which comprises contacting formaldehyde or a source of formaldehyde with a carboxylic anhydride in the presence of a catalyst comprising mixed oxides of vanadium and phosphorous, and optionally containing a third component selected from titanium, aluminum, or preferably silicon. In Ind. Eng. Chem. Res., Vol. 36, No.11, 1997, 4600-4608, Spivey et al. report that the highest yields of methacrylic acid were obtained with the Vanadium-Silicon-Phosphorous ternary oxide catalyst with V--Si--P atomic ratio of 1:10:2.8.
In U.S. Pat. No. 5,808,148, Gogate et al. disclose a process for the preparation of alpha,beta-unsaturated carboxylic acids and esters which comprises contacting formaldehyde or a source of formaldehyde with a carboxylic acid, ester, or a carboxylic acid anhydride in the presence of a catalyst comprising an oxide of niobium. The optimum catalyst in the catalytic synthesis of methacrylates comprised a mixed niobium oxide-silica composition containing 10% Nb2O5 (Ind. Eng. Chem. Res., Vol. 36, No.11, 1997, 4600-4608; Symposium Syngas Conversion to Fuels and Chemicals, Div. Pet. Chem., Inc., 217.sup.th National Meeting, American Chemical Society, Anaheim, Calif., 1999, 34-36).
In a related approach to synthesizing methyl methacrylate, the synthesis of isobutyric acid is followed by oxidative dehydrogenation to yield methacrylic acid, which is then esterified with methanol to yield methyl methacrylate. The key technical challenge lies in the selective oxidative dehydrogenation of isobutyric acid to methacrylic acid and three classes of catalysts have been disclosed: 1) iron phosphates, 2) vanadium-phosphorous mixed oxides or with a ternary component, and 3) heteropolyacids based on phosphomolybdic acid.
In Catalysis Reviews, Sci. Eng. 40(1&2), 1-38, (1998), Millet presents a comprehensive review of iron phosphate catalysts disclosed in the patent literature. According to Millet, the optimum catalysts for this process have P/Fe ratio greater than 1.0, are promoted with alkali metals, silver or lead, and may be supported on silica or alundum. The reaction is conducted at 365.degree. to 450.degree. C. in the presence of oxygen and a co-feed of up to 12 moles H.sub.2 O per mole isobutyric acid is needed to generate a catalyst with high activity. In Applied Catalysis A: General, 109 (1994) 135-146, Ai et al. further discussed the role of many different promoters for iron phosphate catalyst and found that the best performance was obtained with Pb.sup.2+.
In Journal of Catalysis 98, 401-410 (1986), Ai found that V.sub.2 O.sub.5 --P.sub.2 O.sub.5 binary oxide catalysts were effective for the synthesis of methacrylic acid by oxidative dehydrogenation of isobutyric acid. The selectivity to methacrylic acid was a maximum for catalysts with P/V ratio in the range 1.0 to 1.6 when tested in the temperature range 190.degree. C. to 280.degree. C. Ai also disclosed that these catalysts are selective in the vapor phase aldol condensation of (1) formaldehyde with propionic acid to produce methacrylic acid (Appl. Catal., 36 (1988) 221-230; J. Catal. 124, (1990) 293-296) and (2) formalin with acetic acid to produce acrylic acid (J. Catal. 107, (1987) 201-208).
In Journal of Catalysis 124 (1990) 247-258, Watzenberger et al. describe the oxydehydrogenation of isobutyric acid with heteropolyacid catalysts, such as H.sub.5 PMo.sub.10 V.sub.2 O.sub.40.
In U.S. Pat. No. 4,442,307, Lewis et al. disclose a process for the preparation of formaldehyde by oxidizing dimethyl ether in the presence of a catalyst comprising oxides of bismuth, molybdenum and iron. Table 1 of said patent provides the only illustrative Examples at 500.degree. C. in which a 54%Bi-24%Mo-2%Fe catalyst afforded 42% conversion and 46% formaldehyde selectivity while a 55%Bi-25%Mo catalyst gave 32% conversion and 28% formaldehyde selectivity.
Selective catalytic C--C bond formation on MgO to produce .alpha.,.beta.-unsaturated compounds was described by Korukawa et al. (Heterogeneous Catalysis and Fine Chemicals, Guisnet et al. Eds., Elsevier Science Publishers, 1988, 299-306). The authors claim to have developed a novel synthetic route by using MeOH as a methylenylating agent. The synthetic method uses magnesium oxide catalysts activated by transition metal cations to produce formaldehyde. According to the authors, "methyl or methylene groups at .alpha.-position of saturated ketones, esters or nitrites are converted to vinyl groups by the C--C bond formation using methanol as a CH.sub.2.dbd. source." The reaction of methyl propionate with methanol over manganese-promoted MgO afforded 10% conversion of methyl propionate and produced 60% methyl methacrylate (MMA), 18% methyl isobutyrate (MIB) and 22% ketones. The reaction occurs at 400.degree. C. in the absence of O.sub.2 and co-produces H.sub.2 and H.sub.2 O.
Despite the foregoing developments, it would be desirable to provide a low cost, ethylene-based route through propionic acid to methyl methacrylate employing catalysts that can operate at low temperatures to avoid excessive oxidation of the methylating agent to carbon oxides.
In addition, it would be desirable to provide a method for methylating a carbon adjacent to an electron withdrawing group.
All references cited herein are incorporated herein by reference in their entireties.